Leather has enjoyed widespread popularity and usage for centuries. It has been, and currently is, widely used for footwear, garments of various types, luggage, upholstery, sporting goods, and a variety of other products and goods.
Leather is produced from animal hides or pelts which are composed primarily of a three dimensional network of protein fibers. The proteins include collagen, keratin, elastin, and reticulin. Collagen is the leathermaking protein of the hide. Collagen is responsible for the great strength and toughness of the hide and of the leather produced from the hide.
The hide of animals is composed of an interwoven fibrous mat, a thin cellular outer layer supporting hair, and a fatty layer which attaches to the underlying muscles of the animal. The outer layer is normally referred to as the grain layer. The corium layer is beneath the grain layer and is composed primarily of bundles of collagen fibers which are interwoven in a random three dimensional pattern.
Many separate steps are involved in the processing of hides to produce useful leather for the production of sundry types of goods and garments. The hides may be provided to the tannery in a brine-cured condition. The curing dehydrates the hide. After the salt has been removed, the fibers are rehydrated by soaking. A detergent may be added to speed the hydration. Soaking removes water-soluble protein and cleans the hide. Even if the hides are not brine-cured prior to delivery to the tannery, it is still necessary to soak the hides when they are received.
Then the hair (keratin) can be removed using a saturated solution of calcium hydroxide (lime) by itself or in combination with sodium sulfide or sodium sulfhydrate. The lime loosens the hair for easy removal. Hair can also be removed by dissolving it with sulfide at high pH. If desired, the hair (e.g., wool) may be left on the hide and not removed.
The hide may be soaked in fresh lime solution to open up the collagen fiber structure and remove additional proteins. This allows better penetration of tanning chemicals.
Then the hide is ready for deliming (if a liming step has been used), bating and pickling to prepare the hide for tanning. Limed hide has a high pH (e.g., about 12). The hide is washed in water to remove soluble lime and loose hair particles. Ammonium sulfate is commonly used as a deliming salt.
Following the deliming step, if used, the hide is subjected to bating, which is the use of enzymes to break down miscellaneous proteins in the hide. Detergents may be added to assist in fat removal. After bating, the hide is pickled with sulfuric acid to lower the pH. The hide must be in an acid condition for the tanning operation.
The tanning operation involves the treatment of the hide to preserve it and form useful leather. Chrome tanning salts are well known and widely used for this purpose. Chrome sulfate in particular is a common salt used in tanning. Other types of tanning agents can also be used, if desired, such as vegetable tanning agents (i.e., polyphenolic compounds), mineral tanning agents (i.e., zirconium, aluminum, iron, silica), resin tanning agents, oil tanning, sulfonyl chloride, or aldehydes (i.e., formaldehyde and glutaraldehyde).
After the hide has been tanned, it may be retanned, dyed and fatliquored. Retanning procedures allow subsequently applied dye to better penetrate the leather. Fatliquoring is the application of oil-in-water emulsions to the leather. It may be done simultaneously with the dyeing process. Surfactants are normally used in such emulsions. The fatliquoring process is for the purpose of putting oil into the hide to lubricate the fibers. This improves the appearance and also the physical properties of the leather.
After fatliquoring, the leather can be dried. This involves the removal of excess water and completes the reactions of some of the materials used to treat the leather.
Thus, many steps and processing techniques are required to convert natural animal hides or pelts into the desired leather products. Each of the various steps involves a considerable amount of time and specialized treating baths.
Because of the great popularity and demand for high quality leather products, the processing of the hides and pelts must be performed carefully and efficiently in order to produce the best quality leather possible. Consequently, there is always a desire to speed the processing and to increase the quality in the resulting product.